(1.) Field of the Invention
The present invention relates to a process for removing sulfur oxides from a sulfur oxide-containing gases at relatively low temperatures between about 100.degree. C. and 400.degree. C. In particular, the invention relates to the use of layered double hydroxide (LDH) sorbent compositions in the desulfurization of sulfur-containing gases from the flue gases cold side of coal-burning power plants.
(2.) Prior Art
In fossil-fuel-fired power plants, the sulfur content of the feed coal is oxidized during combustion to sulfur oxides (SO.sub.2 and SO.sub.3, commonly referred to as "SO.sub.x "), which are released through stacks to the atmosphere, and are responsible for deposition as "acid rain". Analyses of flue gas produced by power plants burning coal before desulfurization, show 0.5% -0.2% SO.sub.2 and about 0.005% SO.sub.3. Control of SO.sub.x emission is mandated by the US Environmental Protection Agency (EPA), and various studies are under way to develop methods for its removal from flue gas streams.
Formation of SO.sub.x in combustion processes can be reduced by modifying the burner design and combustion system, by changing the operating conditions and by using fuels with lower sulfur contents. The most popular and inexpensive method of reducing SO.sub.x emission is the addition of reactive dry sorbents with the fuel. Accordingly at present, SO.sub.x removal is most often accomplished by using lime (CaO) or lime stone (CaCO.sub.3). Several other basic sorbents like MgO, ZnO also are found to be effective in removing SO.sub.x . For a review on dry sorbents see for example, Komppa, V., "Dry Adsorption Processes for Removal of SO.sub.x and NO.sub.x in Flue Gases--a review," Paperii ja Puu, 5, 401 to 405 (1986).
Use of Group 2 (formerly Group IIA) metal oxides such as magnesium and calcium oxides as SO.sub.x sorbents has been disclosed in several patent disclosures and recent examples include U.S. Pat. Nos. 3,835,031 and 3,699,037. Several other metal oxides of varying degrees of effectiveness as SO.sub.x sorbents are described in U.S. Pat. No. 4,153,534 which include oxides such as sodium, scandium, titanium, iron, chromium, molybdenum, manganese, cobalt, nickel, copper, zinc, cadmium, rare earth metals, and lead.
In typical coal-fired power plants the ground sorbent, for example lime or limestone, is added into boilers along with coal or sprayed into towers as a slurry to contact the flue gas. The SO.sub.2 reacts with calcium hydroxide to form a calcium sulfite slurry which is then partially oxidized with air to calcium sulfate. In this way the sulfur oxides are retained as harmless solid compounds which can be removed from the stack gas by electrostatic precipitation or other standard methods. This hot side dry sorbent injection is potentially attractive for retro-fitting existing power plants since no major structural alterations are required. U.S. Pat. Nos. 5,116,587, 5,114,898 and 5,114,691 to Pinnavaia and others describe the use of LDHs at temperatures above about 400.degree. C. to remove SO.sub.x.
Another evolving technology of flue gas cleanup involve the post-combustion sorbent injection. In this post-combustion gas cleanup, the sorbents are injected downstream of the boiler to capture SO.sub.2. This cool-side desulfurization technology make use of the existing or modified duct work and particulate collection system (electrostatic precipitator or bag house) as the SO.sub.2 capture zones, resulting in a low installed capital cost and short construction time relative to conventional wet scrubbing (FGD). Cool-side desulfurization, therefore, would be particularly well-suited for retrofit applications in coal burning power plants that lack adequate space for FGD installation or that have limited remaining boiler life.
Several calcium and sodium-containing sorbents have been evaluated for cool-side flue gas desulfurization process during the past few years. The publication by C Jorgensen, et. al., in Environmental Progress, vol. 6, pp. 26-32 (1987) has evaluated several of these sorbent compositions. Among these sorbents, the sodium containing compounds such as sodium bicarbonate and sodium carbonates were found to be highly effective in capturing SO.sub.x from flue gas streams. In addition, several pilot-scale commercial processes are presently under way to develop suitable methods for desulfurization of flue gases from coal-burning power plants. These processes include, injection of sorbents such as lime, hydrated lime, lime stone or their mixtures with sodium-containing sorbents such as sodium bicarbonates [lime dual alkali (LDA) or limestone dual alkali (LSDA) processes]. These sorbents are injected as dry sorbents into the filter bag house or to the duct work upstream of bag house (duct sorbent injection, DSI) .
LDHs are a group of anionic clay minerals. These have positively charged sheets of metal hydroxides, between which are located anions and some water molecules. Most common LDHs are based on double hydroxides of such main group metals as Mg, and Al and transition metals such as Ni, Co, Cr, Zn and Fe etc. These clays have structure similar to brucite [Mg(OH).sub.2 ] in which the magnesium ions are octahedrally surrounded by hydroxyl groups with the resulting octahedra sharing edges to form infinite sheets. In the LDHs, some of the magnesium is isomorphously replaced by a trivalent ion, such as Al.sup.3+. The Mg.sup.2+, Al.sup.3+ OH layers are then positively charged, necessitating charge balancing by insertion of anions between the layers.
One such anionic clay is hydrotalcite in which the carbonate ion is the interstitial anion, and has the idealized unit cell formula [Mg.sub.6 Al.sub.2 (OH).sub.16 ](CO.sub.3).4H.sub.2 O and referred to as Mg.sub.3 Al --CO.sub.3. However, the ratio of Mg/Al in hydrotalcite-like can vary between 1.7 and 4 and various other divalent and trivalent ions may be substituted for Mg and Al. In addition, the anion which is carbonate in hydrotalcite, can be replaced in synthesis by a large number of simple anions such as NO.sub.3.sup.-, Cl.sup.-, OH.sup.-, SO.sub.4.sup.2- etc. These LDHs, based on their structure, fall into the Pyroaurite-Sjogrenite group, where brucite-like layers carrying a net positive charge alternate with layers in which the oxygen atoms of carbonate groups and water molecules are distributed on a single set of sites.
Hydrocalumite and related synthetic compounds also have a layered structure in which positively charged metal hydroxide layers alternate with the interlayers containing anions and water. The hydroxide layers contain specific combinations of metal ions derived from on one hand divalent calcium cations and on the other from trivalent cations of metals such as iron, or more particularly, aluminum. The interlayers contain anions such as OH.sup.2, SO.sub.4.sup.2-, Cl.sup.-, NO.sub.3.sup.- and, in particular CO.sub.3.sup.2-. The general formula for the group is [Ca.sub.2 M.sup.3+ (OH).sub.6 ]X.yH.sub.2 O, where M.sup.3+ is a tripositive ion and typically Al.sup.3+, X is a singly charged anion or equal amounts of more highly charged ones, and y is between 2 and 6. As in the Pyroaurite-Sjogrenite group, principal layers alternate with interlayers, the principal layers having the composition [Ca.sub.2 M.sup.3+ (OH).sub.6 ].sup.+ and the inter-layers consisting of water molecules and anion X. However, because of the difference in size between the Ca.sup.2+ and Al.sup.3+ ions, the M.sup.2+ :M.sup.3+ ratio is fixed at 2:1 and their arrangement is ordered. The only known natural mineral in the group is hydrocalumite the composition of which is approximately [Ca.sub.2 Al(OH).sub.6 ](OH).sub.0.75 (CO.sub.3).sub.0.125.2.5H.sub.2 O. However, there are many synthetic analogous such as [Ca.sub.2 Fe(OH).sub.6 ](SO.sub.4).sub.0.5.3H.sub.2 O, [Ca.sub.2 Al(OH).sub.6 ](OH).6H.sub.2 O etc.
The syntheses of LDHs are generally simple, and the so called "precipitation method" is most popular. If a carbonate-containing product is desired, then the aqueous solution of magnesium and aluminum salts, i.e., nitrate, or chloride, is added to a aqueous solution of sodium hydroxide-carbonate with good mixing at room temperature. The resulting amorphous precipitate is then heated for several hours at 60.degree.-200.degree. C. to obtain a crystalline material. Washing and drying complete the synthesis in quantitative yield. By employing this precipitation method, replacement of all or part of Mg.sup.2+ with other M.sup.II ions such as Ca.sup.2+, Zn.sup.2+, Cu.sup.2+ etc., or replacement of Al.sup.3+ with other M.sup.III ions such as Fe.sup.3+, Cr.sup.3+ etc., is also possible.
Another important aspect of the synthesis of these materials is the variation of the nature of the interstitial anion. The preparation of hydrotalcite-like materials with anions other than carbonate in pure form requires special procedures, because LDH incorporates carbonate in preference to other anions. Most of the time the smaller anions are introduced to the LDH structure, via the precipitation method by using the desired aqueous anion solutions instead of carbonates. However, in these methods the syntheses have exclusively carried out in aqueous solutions under an anaerobic condition to prevent carbonate contamination from the atmospheric carbon dioxide.
Process for the synthesis of hydrotalcite-like clays also have been the subject of a number of patents. Miyata et. al. in U.S. Pat. Nos. 3,796,792, 3,879,523, and 3,879,525 describes hydrotalcite-like derivatives with both cationic layer and anionic substitution including the smaller transition metal anions like CrO.sub.4.sup.2-, MoO.sub.4.sup.2-, and Mo.sub.2 O.sub.7.sup.2-. Both composition and preparative methods are described, and the compositions are said to be useful for catalytic purposes, absorbents, desiccants and the like. Synthetic hydrotalcite-like derivatives with small anions, including anions of transition elements, and also large organic anions such as long chain aliphatic dicarboxylates, are shown to catalyze aldol condensation effectively.
Incorporation of larger anions, such as transition metal polyoxoanions in to the LDH gallery is not easy. This require ion-exchange techniques subsequent to the LDH synthesis. Pinnavaia and Kwon in J. Am. Chem. Soc., 110, 3653 (1988) have demonstrated the pillaring of several polyoxometallates including V.sub.10 O.sub.28.sup.6- into the hydrotalcite structure containing Zn and Al metal ions in the layers. In U.S. Pat. No. 4,452,244 by Woltermann disclosed the preparation of several polyoxometallate-LDHs. However, no XRD or analytical data were given to establish the purity of those materials. Recently, U.S. Pat. No. 4,774,212 by Drezdon and U. S. Pat. No. 5,079,203 by Pinnavaia have disclosed the preparation of several Mg/Al and Zn/Al hydrotalcite-like materials containing transition metal polyoxoanions. Several other methods of preparation of LDHs have been described in prior art publications, particular reference being made to following review journal articles by S. L. Suib et. al., in Solid State Ionics, 26, 77 to 86 (1988), and W. T. Reichel in CHEMTECH, 58 to 63 (1986). The exchange of gallery anions with other anions are exclusively done in aqueous solutions. All these ion exchange reactions have been performed in aqueous solutions. In this disclosure we perform solid-gas reactions involving hydrotalcite-like LDHs and sulfur containing gas streams, to replace gallery anions, especially CO.sub.3.sup.2- anions from the LDHs, with sulfur-containing anions.
The nature of the thermal decomposition of LDHs, especially the hydrotalcite-like materials, have been studied in detail. For example, upon thermolysis, hydrotalcite [Mg.sub.6 Al.sub.2 (OH).sub.16 ](CO.sub.3).4H.sub.2 O loses weight in two stages. First, it loses the four interstitial water molecules when heated to 200.degree. C., while retaining the skeletal hydroxide and the interlayer carbonate. Additional heating from 275.degree. C. to 450.degree. C. results in the simultaneous loss of hydroxyl groups and carbonate as water and carbon dioxide, respectively. Reichle in J. Catal. 101, 352 to 359 (1986) has shown that this heating of hydrotalcite was accompanied by an increase in the surface area from about 120 to about 230 m.sup.2 /g (N.sub.2 /BET) and a doubling of pore volume (0.6 to 1.0 cm.sup.3 /g, Hg intrusion). Further heating to higher temperatures causes lowering of surface area as well as reactivity. At 1000.degree. C., the formation of MgO and the spinel phase, MgAl.sub.2 O.sub.4 has been observed.
Thus, the thermal decomposition of LDHs leads to the formation of active metal oxides with fairly high basic character (pKa .ltoreq.35) and high surface area. These thermally treated materials have exceptionally well-dispersed reactive metal centers, and these properties of LDHs have been utilized to develop new class of sorbents which performed as efficient sorbents for the flue gas desulfurization. The results of this work have been disclosed in our U.S. Pat. Nos. 5,116,587, 5,114,898 and 5,114,691. These patent disclosures specifically describe the use of transition metal promoted LDH compositions that are suitable for the hot-side flue gas desulfurization. The transition metals present in these sorbents oxidize SO.sub.2 in flue gas at boiler temperatures to more reactive SO.sub.3. These molecules showed better SO.sub.x absorptivities at temperatures above 500.degree. C. and were an attractive class of sorbents for the furnace sorbent injection process to remove sulfur-containing gases. The transition metals that are capable of oxidizing SO.sub.2 to SO.sub.3 when incorporated into the LDH structure as layer ions or as gallery anions were disclosed in U.S. Pat. Nos. 5,079,203, 5,114,691, and U.S. Pat. Nos. 5,116,587 and 5,114,898, respectively. During the reaction of these LDHs at elevated temperatures, the basic M.sup.2+ metal site, react with SO.sub.3 to form thermally stable harmless metal sulfates MSO.sub.4.
The sorbents described above are examples of "disposable" SO.sub.x sorbents. The SO.sub.x components in the spent sorbents are in the form of non hazardous refractory sulfate form such as CaSO.sub.4 or MgSO.sub.4 and thus suitable for the safe deposition. An economically attractive sorbent compositions would be the one that can be recycled. Recyclable absorbents would allow entrapped SO.sub.x to be released from the spent sorbents and isolate fresh sorbents for reuse, thus eliminating the need for landfill disposal. The released SO.sub.x can be used for sulfuric acid manufacture.
European Patent Application EP-A 278 535 has recently, described a catalyst composition suitable for the refining of heavy sulfur- and metal-containing petroleum feeds. Thus, the catalyst composition according to the disclosure contained a catalytically active zeolitic material such as ZSM-5, ZSM-11 etc. for the conversion of hydrocarbons, an anionic clay material with an LDH structure for the binding and removal of sulfur oxides, and a matrix material such as kaolin or alumina. Preferred catalyst compositions contained 1 to 30 percent amounts of anionic clay compositions, based on total catalyst composition. These sorbents remove sulfur oxides from the gas streams of fluidized catalyst cracking (FCC) units operating at temperatures around 700.degree. C. However, they are inefficient for rapidly capturing SO.sub.x from the flue gas of coal-burning power plants owing in part to the short contact time between the adsorbent and the flue gas.